We have reasonably hard water -- municipal well sourced along with
CAP.

I've plumbed the "outdoor water supplies" (i.e., irrigation and hose
bibbs) to use water directly from the municipal supply (i.e., NOT
"locally" treated) as I don't want to be paying to "treat" water
that's going out a garden hose, etc.

[I don't think the plants would like the extra salt, regardless]

There are ~20 solenoid operated valves buried around the yard
(little concrete, covered vaults that I built for them). Each
operates an irrigation "zone" *or* one of the 4 hose bibbs
scattered around the perimeter.

The irrigation *lines* are all below grade. So, essentially safe
from freezes. But, the hose bibbs require the water supply
(exiting the solenoid valve) to climb to a height that is
convenient for a human user to access. So, that portion of
the plumbing that is above grade is exposed to the elements
more than the rest of the lines which remain below grade.

These risers are 3/4" copper run through the "voids" in 8"
"half (cinder) blocks" which have then been filled with
"Nasty Foam" (TmReg) to improve insulation.

So, you have a length of copper pipe that exits the solenoid valve,
rises through the surface of the soil *into* this foam-filled
void and terminates at a hose bibb -- the valve of which might be
open or closed

[no idea as to which; the whole point of the solenoid is to allow
me to put a garden hose on a bibb, set the (manual) valve to an
appropriate flow rate, and direct the hose's output to a
particular spot in the yard that needs "supplemental watering".
Then, use the solenoid to turn that water supply on or off.]

Of course, I don't want to risk the water standing in that pipe
freezing and rupturing the pipe (I can't rely on it destroying
the valve, instead : ).

To that end, I installed these auto-drain valves (colloquially
known as "spitters") on the "load" side of the solenoid.
They resemble a PCV valve in construction: an opening which
can be plugged by a slug of metal inside the valve body.
A low tension spring holds the plug AWAY from the opening
so it is normally open. The slightest water pressure acts
to push the slug (plug) against the hole thereby sealing it.

When pressure is removed, the spring's force eventually
overcomes that of the fading water pressure and the slug
exposes the opening. As this is located at the low point
in the plumbing, the water "standing" above it *should*
drain out through this opening.

[This is a sort of *******ization of their use]

Over time, I suspect these will fail due to concretions
from the water supply. Anyone know for a fact if this is
true? I've located them in a way that allows me to
service them down the road. But, I'm looking for FEWER
chores, not MORE! :

Alternatively, I'm considering forcing all of the solenoids
*open* (after turning off the main supply) and hoping water
will "seek its level" throughout the system -- instead of
dealing with each of these little "stand pipes" in isolation.

[Or, even adding an "extra" valve whose sole purpose is to be
the "low point" in the system so there is a place BELOW GRADE
for all water to drain]